Radio navigation systems are used for providing useful geographic location information to aircraft pilots, mariners, and even land-based vehicles such as trucks, buses and police vehicles. Early radio navigation systems used transmitter signpost techniques which rely on phase and timing information transmitted by several terrestrial, geographically separated transmitters. One common radio navigation system currently in use is Loran which also relies on land-based transmitters to provide the navigation signals. The latest radio navigation system is referred to as the Global Positioning System (GPS), and it is maintained by the government of the United States of America.
The GPS navigation system relies on satellites which are constantly orbiting the globe. When the system is fully operational, any user of GPS, anywhere on the globe, will be able to derive precise navigation information including 3-dimensional position velocity and time by day. The system is expected to become fully operational in 1988 with 18 satellites in orbit. Navigation fixes using GPS are based on measurements of propagation delay times of GPS signals broadcast from the orbiting satellites to the user. Normally, reception of signals from 4 satellites is required for precise location determination in 4 dimensions (latitude, longtitude, altitude, and time). Once the receiver has measured the respective signal propagation delays, the range to each satellite is calculated by multiplying each delay by the speed of light. Then the location and time are found by solving a set of four equations incorporating the measured ranges and the known locations of the satellites. The highly precise capabilities of the system are maintained by means of on-board atomic clocks for each satellite and by ground tracking stations which continuously monitor and correct satellite clock and orbit parameters.
Each GPS satellite transmits two direct-sequence-coded spread spectrum signals at L-band: a L1 signal at a carrier frequency of 1.57542 GHz, and a L2 signal at 1.2276 GHz. The L1 signal consists of two phase-shift keyed (PSK) spread spectrum signals modulated in phase quadruature: the P-code signal (P for precise), and the C/A-code signal (C/A meaning coarse/acquisition or clear/access). The L2 signal contains only the P-code signal. The P and C/A codes are repetitive pseudorandom sequences of bits (called "chips" in spread spectrum parlance) which are modulated onto the carriers. The clocklike nature of these codes is utilized by the receiver in making time delay measurements. The codes for each satellite are distinct, allowing the receiver to distinguish between signals from the various satellites even though they are all at the same carrier frequency. Also modulated onto each carrier is 50 bit/sec data stream (also distinct for each satellite) which contains information about system status and satellite orbit parameters, which are needed for the navigation calculations. The P-code signals are encrypted, and available only to classified users. The C/A signal is available to all users.
The operations performed in a GPS receiver are for the most part typical of those performed in any direct-sequence spread spectrum receiver. The spreading effect of the pseudorandom code modulation must be removed from each signal by multiplying by a time-aligned, locally-generated copy of it's code, in a process known as despreading. Since the appropriate time alignment, or code delay, is unlikely to be known at receiver start-up, it must be searched for during the initial acquisition stage. Once found, proper code time-alignment just be maintained during the "tracking" phase of receiver operation, as the user moves about. A mechanism for providing this alignment is called a delay-locked loop.
Once despread, each signal simply consists of a 50 bit/sec PSK signal at some intermediate carrier frequency. This frequency, is somewhat uncertain due to the Doppler effect caused by relative movement between satellite and user, and to receiver local clock error. During initial signal acquisition the Doppler frequency must be searched for, since it is usually unknown prior to acquisition. Once the Doppler frequency is approximately determined, carrier demodulation proceeds using a local carrier signal derived from either a squaring or a Costas carrier recovery loop. In order to maintain at a constant level the dynamic charcteristics of the carrier recovery and delay-locked loops as the signal strength varies, GPS receivers are usually provided with automatic gain controlled (AGC).
After carrier demodulation, data bit timing is derived by a bit synchronization loop and the data stream is finally detected. A navigation calculation may be undertaken once the signals from 4 satellites have been acquired and locked onto, the necessary time delay and Doppler measurements have been made, and a sufficient number of data bits (enough to determine the GPS "system" time and orbit parameters) have been received.
To accomplish the functions described above, all known GPS receivers utilize standard analog technology with a limited amount of digital processing in the "back-end" of the receiver. The navigation computations are typically performed using a microprocessor, which is well-suited to the task. In addition, certain baseband functions, such as data detection, bit timing recovery, and some Costas loop processing, are performed digitally in known receivers. However, all pseudorandom code despreading, carrier demodulation, delay-locked loop processing, and gain control are implemented using analog components.
An example of a prior GPS receiver is shown and described in a paper by Chao, Low Cost RF/LSI Technologies for Commercial GPS Receivers, MICROWAVE SYSTEMS APPLICATIONS TECHNOLOGY CONFERENCE, March 1983. Another example of a prior GPS receiver is shown and described in a paper by Yiu, Crawford, and Eschenback, A Low-Cost GPS Receiver for Land Navigation, JOURNAL OF THE INSTITUTE OF NAVIGATION, Fall 1982.
There are several disadvantages to the processing approach used in prior GPS receivers. Due to the complex nature of the GPS signal and the complicated processing required, these receivers typically require large numbers of discrete components or highly specialized analog integrated circuits, resulting in high manufacturing cost. This is especially true if the receiver is designed to process the four required satellite signals simultaneously, since the circuitry for one "channel" must be duplicated three times. To reduce circuit complexity, some receivers employ what is known as sequential processing, where the hardware for one channel is time-shared among the four incoming signals. Receiver performance is degraded with this technique, however, since three-quarters of the information in each signal is lost.
Furthermore, conventional receivers suffer from the usual problems found with analog designs, for example, degradation in performance due to aging, temperature/humidity variations, and to mismatches in certain signal path characteristics.
Another drawback to the processing methods employed in current GPS receivers is the long time needed for initial signal acquisition. As mentioned above, before the four satellite signals can be tracked they must be searched for in a two-dimensional search "space", whose dimensions are code delay and Doppler frequency. Typically, if there is no prior knowledge of a signal's location within this search space, as would be the case of a receiver "cold start", a large number of code delays (about 2000) and doppler frequencies (about 15) must be searched. Thus, for each signal, up to 30,000 locations in the search space must be examined. Typically these locations are examined one-at-a-time sequentially, a process which can take up to 5 to 10 minutes. The acquisition time is further lengthened if the identities (i.e., codes) of the four satellites within view of the receiving antenna are unknown. Methods to shorten the acquisition time have been devised but are quite expensive to implement. One technique, for example, employs surface acoustic wave filters matched to each of the 18 satellite codes to effectively perform the despreading. Another technique utilizes multiple conventional despreading circuits operating in parallel in order to search several code delays simultaneously.
It is apparent from the above discussion that conventional GPS receivers, which rely mainly on analog technology, suffer from many disadvantages. By contrast, a receiver based substantially on digital technology would be free, potentially, from many of these problems. Unfortunately, due to the wide bandwidth of the GPS signals to be received, straight-forward application of digital signal processing principles to current receiver processing methods would result in an extremely costly, high power consumption, device. Therefore, the need exists for a GPS receiver architecture which is amenable to a digital implementation.